The Relationship between Protective Glue and Platemaking Quality

The printing graphic part of the printing plate has the feature of lipophilic and water repellent; on the contrary, the non-graphic part is hydrophilic and oil-repellent. Therefore, the quality of the printing plate plate will directly affect the quality of the offset printing product. The blank part of the positive PS plate is The metal oxide Al2O3 and its crystal water are high-energy surfaces (0.7J/m2). It can be wetted by the fountain solution during printing. It is easy to react with O2, N2, and CO2 in the air. It is easy to generate oxidation. In this case, the surface energy is lowered, the wettability is deteriorated, and impurities, oil stains, etc. are adsorbed, and it is difficult to clean the printed product to cause dirtiness. Therefore, it is necessary to protect the blank portion and utilize the film forming property of the glue (low-energy surface of the polymer compound) to isolate the plate from the air, prevent the plate from scratching, protect from light, ensure the hydrophilicity of the blank portion itself, and give The blank part has better hydrophilic properties. This article provides some references for plate making by introducing the methods and precautions for using the protective glue.

First, the composition and function of protective plastic

1. Composition

A transparent glue made of natural glue or synthetic glue, which can protect the printing plate graphic part and the non-graphic part to prevent the plate from oxidizing a dirty liquid. The commonly used protective glues are gum arabic and new type of high-molecular glue (the protective glue used by the author is gum arabic, and this article also takes it as an example).

2. Features

1 hydrophilic. The molecular structure of gum arabic consists of -COOH and -OH attached to the same hydrocarbon bond. These two functional groups are hydrophilic and the hydroxyl group (-OH) is more likely to accept moisture than the carboxyl group (-COOH). So when the gum arabic is dissolved in water, it will dissolve in the process of diffusion-absorption-swelling for about 72 hours to form a molecular solution of the dispersion system. The hydroxyl groups in each molecule are extremely hydrophilic and can be in water. Automatically dissolves into a single-phase stable system that neither condenses nor precipitates, ensuring long-term dispersion stability.

2 weakly acidic. Peach gum contains arabic acid (XCOOH), its aqueous solution is weakly acidic, and it reacts with metal or metal oxides to produce arabinates in the presence of alkali and releases hydrogen gas, which has good adsorption.

3 perishable. In the hot environment (Arabica gum use temperature is -5 °C ~ 45 °C), Yeast yeast is easy to breed and ferment in gum arabic, free arabinic acid, change the gel. This is the general nature of sugars, and it is also the reason that gum arabic and long-term use will cause rancidity. As the temperature rises, the acidity increases, and the corrosiveness of the metal printing plate increases, and it is easy to lose edition. Al2O3 falls off. This is especially important in the summer, keeping the protective rubber away from heat.

3. Role

Improve the hydrophilicity of non-graphical parts and prevent dirt on the prints. Protect the layout and prevent scratches.

Second, the reasons for the use of protective rubber in accordance with the provisions of the ratio:

Take the school-run factory protective glue as an example, dilute it with water in different proportions and test its specific gravity.

According to the test situation in Table 1, draw the diagram.

1. The relationship between the specific gravity and the dilution rate can be seen: as the dilution rate of the glue increases, its specific gravity decreases, reaching a certain proportion and its specific gravity is close to 1 (the proportion of water).

2. The relationship between dilution rate and quality

We also do this kind of experiment: According to the different proportion disposition, see the quality situation of the edition material after gluing.

As can be seen from Figure 2, the proportion of the protective rubber is between 1:1 and 1:3. The quality of the PS plate is the best. Therefore, it can be seen that the specific proportion of the dilution ratio of the protective rubber varies greatly and should be strictly followed. The proportion of use PS version of protective plastic.

Third, the failure caused by improper use of protective plastic

1. The protective glue is too thick

1 When printing on the machine, the ink is not good

Figure 3 shows the state of the ink on the machine when the protective glue is normal; B is the state of ink on the machine when the protective glue is too thick.

The protective glue is too thick, which results in over-thickness of the sizing layer of the plate after the gluing. When the stencil is on the machine, it is easy to cause the failure of the plate in printing.

Countermeasures: Diluted to the standard glue concentration;

2 before the printing machine, prone to cracking

Figure 4A shows the state of the ink on the protection mask when it is normal; B shows the on-inking condition when the protection glue is too thick.

When the protective gel is too thick, cracking occurs (as shown in Fig. 4C). The black wavy line in the picture is a crack, where the blank space will be

Occurrence of oxidation directly leads to dirty printing on the machine.

Countermeasure: Normal printing after processing with cleaning paste.

2. The protective rubber is too thin

1 Before printing on the machine, it is easy to scratch the damaged layout.

FIG. 5A is a state where the protective ink is applied to the ink in a normal state; B is a state in which the protective ink is overly thinned.

This situation often occurs during plate making. Attention must be paid to the plate making process. At the same time, the operator must pay attention when taking the plate to prevent scratches.

Countermeasures: Add padding paper between the two plates to adjust the glue concentration to avoid scratching the plate. We can make good use of the pad paper in the newly purchased plate.

2 Before the machine is brushed, the spots are dirty.

Fig. 6A shows the state of the ink on the protective film when the protective glue is normal; B indicates the state of the ink on the protective film when the protective glue is too thin. Obviously, it was already oxidized before it came on board. It was not properly protected and the spots were dirty.

Countermeasures: Adjust the glue concentration and print normally after cleaning with paste.

3. When the adhesive layer is not dry, the adhesive layer or the photosensitive layer is peeled off, as shown in FIG.

After the protective glue dries

Countermeasures: Blow dry and stack after glueing.

4. Shutdown on dirty

The reason is that after the printing is stopped for a while, the layout is not coated with a protective glue, and the downtime is too long, which causes the surface of the PS plate to be oxidized and caused by dirt.

Countermeasures: After the machine is shut down, protect the rubber and print it with a cleaning paste.

5. The protective glue drying temperature is too high

When the protective glue is in the normal state, the ink is applied. When the glue drying temperature is too high, the ink is applied.

Countermeasures: Reduce the drying temperature, should be controlled below 70 °C. Our actual temperature control is about 65 °C, do not exceed 70 °C, otherwise it will cause the protective glue curing reaction.

Fourth, the use of protective rubber:

Hand sizing: Use collagen and water to dilute 1:1. A suitable amount of gum (about 45 ml of 1 m2) was poured on the plate surface, and then uniformly coated with a cellulose sponge, and the excess glue was wiped off with a dry cloth and then dried.

Machine gluing: Use collagen and water diluted 1:3. The drying temperature should be controlled below 70°C, generally between 60°C and 65°C.

Note: The glue applicator with no supplemental construction should be replaced every three days. The glue applicator with a supplementary configuration should be changed once a week. The lower the protective colloid concentration, the better the ink repellency at the time of printing, but it is easily bumped and dirty. Conversely, if the concentration of the protective gel is too high, it is likely to cause poor inking.

V. Precautions on the use of protective plastics

The electrical conductivity, specific gravity, and pH of the protective rubber under various conditions are shown in Fig. 9 .

â–  When the protective gel concentration is normal, the protective adhesive's PH value is weakly acidic.

â–  Counterflow to the developer causes a sudden increase in pH.

â–  The countercurrent flow of water reflects the decrease in specific gravity and electrical conductivity and the increase in pH.

To prevent the concentration of the protective glue: Since the moisture in the protective glue is volatile, the glue concentration will be increased, so the protective glue should be diluted or replaced in time.

The use of a degraded protective glue is difficult to remove during printing, and the adhesion of the photosensitive layer on the PS plate is reduced, and the plate is off, and the oxide layer falls off in severe cases. For large graphic text on the ground, the protective glue concentration can be appropriately reduced.

When the protective rubber is not dry, do not touch the printed area in the hand to prevent fingerprints from being printed on the printed material. The protective glue will harm the human body. Use caution when using it. Do not allow the protective rubber to touch the skin, clothing, or splash on the eyes. Inside.

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